Add Markdown diagrams and render hooks for code blocks

You can now create custom hook templates for code blocks, either one for all (`render-codeblock.html`) or for a given code language (e.g. `render-codeblock-go.html`).

We also used this new hook to add support for diagrams in Hugo:

* Goat (Go ASCII Tool) is built-in and enabled by default; just create a fenced code block with the language `goat` and start draw your Ascii diagrams.
* Another popular alternative for diagrams in Markdown, Mermaid (supported by GitHub), can also be implemented with a simple template. See the Hugo documentation for more information.

Updates #7765
Closes #9538
Fixes #9553
Fixes #8520
Fixes #6702
Fixes #9558

markup/goldmark/internal/render/context.go

@@ -0,0 +1,81 @@
+// Copyright 2022 The Hugo Authors. All rights reserved.
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+// http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+package render
+
+import (
+	"bytes"
+	"math/bits"
+
+	"github.com/gohugoio/hugo/identity"
+	"github.com/gohugoio/hugo/markup/converter"
+)
+
+type BufWriter struct {
+	*bytes.Buffer
+}
+
+const maxInt = 1<<(bits.UintSize-1) - 1
+
+func (b *BufWriter) Available() int {
+	return maxInt
+}
+
+func (b *BufWriter) Buffered() int {
+	return b.Len()
+}
+
+func (b *BufWriter) Flush() error {
+	return nil
+}
+
+type Context struct {
+	*BufWriter
+	positions []int
+	ContextData
+}
+
+func (ctx *Context) PushPos(n int) {
+	ctx.positions = append(ctx.positions, n)
+}
+
+func (ctx *Context) PopPos() int {
+	i := len(ctx.positions) - 1
+	p := ctx.positions[i]
+	ctx.positions = ctx.positions[:i]
+	return p
+}
+
+type ContextData interface {
+	RenderContext() converter.RenderContext
+	DocumentContext() converter.DocumentContext
+	AddIdentity(id identity.Provider)
+}
+
+type RenderContextDataHolder struct {
+	Rctx converter.RenderContext
+	Dctx converter.DocumentContext
+	IDs  identity.Manager
+}
+
+func (ctx *RenderContextDataHolder) RenderContext() converter.RenderContext {
+	return ctx.Rctx
+}
+
+func (ctx *RenderContextDataHolder) DocumentContext() converter.DocumentContext {
+	return ctx.Dctx
+}
+
+func (ctx *RenderContextDataHolder) AddIdentity(id identity.Provider) {
+	ctx.IDs.Add(id)
+}